With the advent of large scale integration of electronic components, there has been an ever increasing demand for improved cooling. The improvement required is not only in the form of higher capability, but requires a more uniform temperature across the entire surface being cooled as well as control of the temperature variations thereof. For most of the lower performance applications, air convection cooling has proved to be adequate. However, the high performance applications, that is, where the power density (or flux) is sufficiently high liquid cooling is required. Various liquid immersion cooling schemes have been devised to handle such requirements, but none have proven capable of achieving temperature control by way of cooling customization. This invention sets forth a method of enhancing and controlling the nucleate boiling of the electronic units such as seim-conductor devices in a dielectric coolant. The theory of nucleate boiling as a means to provide cooling is well known in the literature. For example, U.S. Pat. No. 3,301,314 sets forth a method and means for increasing the heat transfer co-efficient between a wall and a boiling liquid. This enhancement was obtained by depositing a low surface energy material, such as polytetrafluoroethyline, within the artificial nucleation sites. This treatment not only causes the artificial nucleation sites to remain active during prolonged steady-state boiling, but also causes the sites to remain unquenched for a remarkably long period of time, when the system is subjected to temperature cycling. The application of coatings of any kind are not particularly applicable to semi-conductor operations and cause, in some situations, adverse affects, such as material incompatibility, etc. The nucleation sites havve been formed in the surfaces to be cooled by punches, which make small conical indentations or, as is set forth in U.S. Pat. 3,454,081, grooves are made on the surfaces to be cooled by an appropriate tool, and the ridges left thereby are bent over to form cavities within the grooves between the ridges. These proposed machining techniques of scratching and bending the tips of ridges are highly undesirable and incompatible with the manufacture of semi-conductor devices.
The conventional nucleate boiling techniques present two basic problems for semi-conductor application; exhibits is the so called hysteresis which is very much amplified due to the highly polished surfaces of semi-conductors, the other is related to the departure point from nucleate boiling, commonly referred to as DNB (Departure from Nucleate Boiling). The hysteresis problem can be described as the lag in the starting of nucleate boiling and the continuation of the natural convection cooling. This lag causes an undesirable increase in temperature of the unit being cooled. The hysteresis problem is also related to the properties of the dielectric coolant, such as fluorocarbon, which has a very low surface tension and exhibbits an enormous wettability of solid surfaces. In fact, this coolant sometimes wets all natural surface cavities and therefore, causes what is called "cavity deactivation," making it impossible for nucleate boiling to start except at high levels of surface superheat. Another problem associated with hysteresis is the fact that it is virtually unpredictable. Naturally, the unpredictability and the magnitude of the overshoot is not tolerable for cooling applications. With regard to the departure point from nucleate boiling, DNB, it is a manageable problem except that there is a continuing demand to improve that point so that higher heat fluxes can be handled without the possibility of getting into the film boiling regime. Actually, many techniques for application in water have been found to be either ineffective for fluorocarbon liquid because of it's low surface tension or not applicable to the surface of semi-conductors, or a combination of both. The foregoing disadvantages have been overcome by drilling holes in the surfaces to be cooled by a high energy beam such as a laser or electronic beam (E Beam). These high energy beams create specially shaped and finished cavities in the surfaces to be cooled, which cavities have much higher probability of surviving total wetting, and thus serve as more reliable nucleation sites. Also, bubbles or vapors generated from these artificial sites will also enter some of the downstream natural or artificial nucleation sites which have been wetted, and cause what is commonly referred to as cavity reactivation. The cross-section and entrance shapes of the high energy beam created cavities is not only easily done on semi-conductor surfaces, but is also unique in the sense that there is no other known technique that can exactly duplicate it. Of course, the high energy beam method of generating nucleation sites can be controlled to produce cavities of different size, depth and pattern so that cooling customization, that is treating the entire chip or a section or corner of a chip as required, can be attained.